4,406 research outputs found
Effect of turbulence on collisional growth of cloud droplets
We investigate the effect of turbulence on the collisional growth of um-sized
droplets through high- resolution numerical simulations with well resolved
Kolmogorov scales, assuming a collision and coalescence efficiency of unity.
The droplet dynamics and collisions are approximated using a superparticle
approach. In the absence of gravity, we show that the time evolution of the
shape of the droplet-size distribution due to turbulence-induced collisions
depends strongly on the turbulent energy-dissipation rate, but only weakly on
the Reynolds number. This can be explained through the energy dissipation rate
dependence of the mean collision rate described by the Saffman-Turner collision
model. Consistent with the Saffman-Turner collision model and its extensions,
the collision rate increases as the square root of the energy dissipation rate
even when coalescence is invoked. The size distribution exhibits power law
behavior with a slope of -3.7 between a maximum at approximately 10 um up to
about 40 um. When gravity is invoked, turbulence is found to dominate the time
evolution of an initially monodisperse droplet distribution at early times. At
later times, however, gravity takes over and dominates the collisional growth.
We find that the formation of large droplets is very sensitive to the turbulent
energy dissipation rate. This is due to the fact that turbulence enhances the
collisional growth between similar sized droplets at the early stage of
raindrop formation. The mean collision rate grows exponentially, which is
consistent with the theoretical prediction of the continuous collisional growth
even when turbulence-generated collisions are invoked. This consistency only
reflects the mean effect of turbulence on collisional growth
Regulation of peripheral inflammation by spinal p38 MAP kinase in rats.
BackgroundSomatic afferent input to the spinal cord from a peripheral inflammatory site can modulate the peripheral response. However, the intracellular signaling mechanisms in the spinal cord that regulate this linkage have not been defined. Previous studies suggest spinal cord p38 mitogen-activated protein (MAP) kinase and cytokines participate in nociceptive behavior. We therefore determined whether these pathways also regulate peripheral inflammation in rat adjuvant arthritis, which is a model of rheumatoid arthritis.Methods and findingsSelective blockade of spinal cord p38 MAP kinase by administering the p38 inhibitor SB203580 via intrathecal (IT) catheters in rats with adjuvant arthritis markedly suppressed paw swelling, inhibited synovial inflammation, and decreased radiographic evidence of joint destruction. The same dose of SB203580 delivered systemically had no effect, indicating that the effect was mediated by local concentrations in the neural compartment. Evaluation of articular gene expression by quantitative real-time PCR showed that spinal p38 inhibition markedly decreased synovial interleukin-1 and -6 and matrix metalloproteinase (MMP3) gene expression. Activation of p38 required tumor necrosis factor alpha (TNFalpha) in the nervous system because IT etanercept (a TNF inhibitor) given during adjuvant arthritis blocked spinal p38 phosphorylation and reduced clinical signs of adjuvant arthritis.ConclusionsThese data suggest that peripheral inflammation is sensed by the central nervous system (CNS), which subsequently activates stress-induced kinases in the spinal cord via a TNFalpha-dependent mechanism. Intracellular p38 MAP kinase signaling processes this information and profoundly modulates somatic inflammatory responses. Characterization of this mechanism could have clinical and basic research implications by supporting development of new treatments for arthritis and clarifying how the CNS regulates peripheral immune responses
Do group dynamics affect colour morph clines during a range shift?
Funded by Strategic Research Area Biodiversity and Ecosystem Services in a Changing Climate (BECC) Lund and Gothenburg Universities Wenner-Gren Foundation EU FP7 Swedish Research Council Royal Swedish Academy of Sciences (KVA) Stiftelsen Anna-Greta and Holger Crafoords Fund Crafoord FoundationPeer reviewedPostprin
A new population of Darwin's fox (Lycalopex fulvipes) in the Valdivian Coastal Range
Indexación: Web of Science; Scielo.Darwin's fox (Lycalopex fulvipes Martin, 1837) is an endemic of the temperate forests of the Coastal Range of southern Chile, that was reported by Charles Darwin in 1834 in southern Chiloé Island (42° S, 74° W; Martin 1837). Initially known exclusively from that island, it was considered both an insular subspecies of the chilla fox (Lycalopex griseus Gray, 1837) (Housse 1953; Clutton-Brock et al. 1976) and a valid species (Martin 1837; Gay 1947; Osgood 1943). In 1990, a mainland population was reported at Nahuelbuta National Park (ca. 450 km north of Chiloé Island, 37° 47′ S, 72° 59′ W; Figure 1a) in sympatry with the chilla and culpeo foxes (Lycalopex culpaeus Molina, 1782) (Jaksic et al. 1990; Medel et al. 1990; Jiménez et al. 1991). This supported its status as a valid species, later confirmed through genetic studies (Yahnke et al. 1996).http://ref.scielo.org/z7mmt
The influence of Fe and Mn content and cooling rate on the microstructure and mechanical properties of A380-die casting alloys
In the present investigation, the microstructure and tensile properties of aluminium die-cast alloys, based on A380, are studied in details as a function of the iron and manganese content and solidification rate. One set of experiments was designed to examine the ?solely? effect of Fe content, which varied from 0.1 up to 1.6 wt % and another set with manganese additions, Mn:Fe ~ 1:2. Three solidification rates corresponding to ~ 10, 25 and 60 ?m in secondary dendrite arm spacing, SDAS, respectively were employed by using the unique gradient solidification technique. Microstructure analysis reveals that at relatively high cooling rates and iron levels, the iron-rich precipitates are suppressed to some extent and the strength is maintained at high levels, but the ductility is gradually decreased. The amount of iron-rich intermetallics does not appear to influence the size and area fraction of porosity and consequently the result suggests that tensile properties have not been adversely affected by porosity level. These results offer additional insight into commonly discussed microstructure features and their role in the determination of the quality and soundness of Al-Si cast alloys
Truthful Facility Assignment with Resource Augmentation: An Exact Analysis of Serial Dictatorship
We study the truthful facility assignment problem, where a set of agents with
private most-preferred points on a metric space are assigned to facilities that
lie on the metric space, under capacity constraints on the facilities. The goal
is to produce such an assignment that minimizes the social cost, i.e., the
total distance between the most-preferred points of the agents and their
corresponding facilities in the assignment, under the constraint of
truthfulness, which ensures that agents do not misreport their most-preferred
points.
We propose a resource augmentation framework, where a truthful mechanism is
evaluated by its worst-case performance on an instance with enhanced facility
capacities against the optimal mechanism on the same instance with the original
capacities. We study a very well-known mechanism, Serial Dictatorship, and
provide an exact analysis of its performance. Although Serial Dictatorship is a
purely combinatorial mechanism, our analysis uses linear programming; a linear
program expresses its greedy nature as well as the structure of the input, and
finds the input instance that enforces the mechanism have its worst-case
performance. Bounding the objective of the linear program using duality
arguments allows us to compute tight bounds on the approximation ratio. Among
other results, we prove that Serial Dictatorship has approximation ratio
when the capacities are multiplied by any integer . Our
results suggest that even a limited augmentation of the resources can have
wondrous effects on the performance of the mechanism and in particular, the
approximation ratio goes to 1 as the augmentation factor becomes large. We
complement our results with bounds on the approximation ratio of Random Serial
Dictatorship, the randomized version of Serial Dictatorship, when there is no
resource augmentation
Plasma Ejection from Magnetic Flares and the X-ray Spectrum of Cygnus X-1
The hard X-rays in Cyg X-1 and similar black hole sources are possibly
produced in an active corona atop an accretion disk. We suggest that the
observed weakness of X-ray reflection from the disk is due to bulk motion of
the emitting hot plasma away from the reflector. A mildly relativistic motion
causes aberration reducing X-ray emission towards the disk. This in turn
reduces the reprocessed radiation from the disk and leads to a hard spectrum of
the X-ray source. The resulting spectral index is Gamma=1.9B^{1/2} where
B=gamma(1+beta) is the aberration factor for a bulk velocity beta=v/c. The
observed Gamma=1.6 and the amount of reflection, R=0.3, in Cyg X-1 in the hard
state can both be explained assuming a bulk velocity beta=0.3. We discuss one
possible scenario: the compact magnetic flares are dominated by e+- pairs which
are ejected away from the reflector by the pressure of the reflected radiation.
We also discuss physical constraints on the disk-corona model and argue that
the magnetic flares are related to magneto-rotational instabilities in the
accretion disk.Comment: The final version, accepted for publication in ApJ Letter
u-RANIA: a neutron detector based on \mu -RWELL technology
In the framework of the ATTRACT-uRANIA project, funded by the European
Community, we are developing an innovative neutron imaging detector based on
micro-Resistive WELL ( -RWELL) technology. The -RWELL, based on the
resistive detector concept, ensuring an efficient spark quenching mechanism, is
a highly reliable device. It is composed by two main elements: a readout-PCB
and a cathode. The amplification stage for this device is embedded in the
readout board through a resistive layer realized by means of an industrial
process with DLC (Diamond-Like Carbon). A thin layer of BC on the copper
surface of the cathode allows the thermal neutrons detection through the
release of Li and particles in the active volume. This technology
has been developed to be an efficient and convenient alternative to the He
shortage. The goal of the project is to prove the feasibility of such a novel
neutron detector by developing and testing small planar prototypes with readout
boards suitably segmented with strip or pad read out, equipped with existing
electronics or readout in current mode. Preliminary results from the test with
different prototypes, showing a good agreement with the simulation, will be
presented together with construction details of the prototypes and the future
steps of the project.Comment: Prepared for the INSTR20 Conference Proceeding for JINS
Variations In Microstructure And 12 Mechanical Propreties Of Cast Aluminum EN AC 43100 Alloy
The microstructure and mechanical properties of a gravity die and sand cast Al-10%Si-0.4%Mg alloy, which is one of the most important and frequently used industrial casting alloys, were examined. Tensile test samples were prepared from fan blades and sectioned through three positions which experienced different cooling rates. Furthermore, the inherent strength potential of the alloy was revealed by producing homogeneous and well fed specimens with a variety of microstructural coarseness, low content of oxide films and micro-porosity defects, solidified in a laboratory environment by gradient solidification technology. The solidification behaviour of the alloy was characterized by thermal analysis. By means of cooling curves, the solidification time and evolution of the microstructure was recorded. The relation between the microstructure and the mechanical properties was also assessed by using quality index-strength charts developed for the alloy. This study shows that the microstructural features, especially the ironrich needles denoted as ?-Al5FeSi, and mechanical properties are markedly affected by the different processing routes. The solidification rate exerts a significant effect on the coarseness of the microstructure and the intermetallic compounds that evolve during solidification, and this directly influences the tensile properties
Lineshape of the thermopower of quantum dots
Quantum dots are an important model system for thermoelectric phenomena, and
may be used to enhance the thermal-to-electric energy conversion efficiency in
functional materials. It is therefore important to obtain a detailed
understanding of a quantum-dot's thermopower as a function of the Fermi energy.
However, so far it has proven difficult to take effects of co-tunnelling into
account in the interpretation of experimental data. Here we show that a
single-electron tunnelling model, using knowledge of the dot's electrical
conductance which in fact includes all-order co-tunneling effects, predicts the
thermopower of quantum dots as a function of the relevant energy scales, in
very good agreement with experiment.Comment: 10 pages, 5 figure
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